During the past few years the vehicle occupant safety belt systems with which most vehicles have been equipped have employed emergency locking retractors. One of the problems with emergency locking retractors is that they permit a significant amount of elongation of the effective length of the belt in a collision due to the tightening of the several turns of the belt on the belt reel. Those turns or loops of the belt that remain on the reel when it is fastened up are comparatively loose, and the very high forces imposed on the belt in a collision are sufficient to tighten them considerably, thereby allowing a length of the belt to withdraw from the belt reel even though the reel is locked against rotation by the emergency locking mechanism.
There have been various proposals for providing some sort of clamp in association with an emergency locking retractor to reduce or eliminate the release of a length of the belt from the reel due to tightening of the remaining loops. Many of the proposals made in the past have been unsatisfactory either because they never fully lock the belt but let it slip or they present a possibility of failure of the belt upon imposition of a very high pull-out force. There have also been belt clamps which seem adequate to withstand the pull-out force reasonably well without belt failure, but they have been difficult to release, an inconvenience for the occupant who has to release the belt by hand each time the device is activated.
The inventor of the present invention has heretofore invented a belt clamp that follows the general principles of many conventional belt clamps but overcomes their possible problems and fully locks the belt in an effective and relatively simple way. In particular, that belt clamp comprises a movable clamping jaw, a fixed clamping jaw and a movable turning roll having a corrugated surface around which the belt wraps in almost a full U-turn. Upon imposition of a pull-out force on the belt above a selected level, the turning roll engages the movable clamping jaw and moves it toward the fixed clamping jaw, thereby to clamp the belt between the jaws. In order to keep the turning roll from cocking when the belt is pulled in an oblique direction (in a direction sideways to the belt length), the turning roll is carried by a U-shaped lever and is supported by a shaft that slides in arcuate guide slots in the frame--hence, the turning roll axis remains parallel to the crosswise profiles of the fixed clamping jaw as it moves in response to the force in the belt and exerts uniform pressure on the belt across its width. Reference may be made to the specification and drawings of the present inventor's U.S. patent application Ser. No. 275,364 filed June 19, 1981, and entitled "Belt Clamps for Vehicle Occupant Restraint Belt Systems" for a full description and illustration of the above-referred to belt clamp.
In many belt clamps heretofore known in the art, the squeezing pressure applied to the belt increases as the tensile load on the belt increases. The squeezing force can reach a level high enough to initiate failure of the belt, particularly in clamps that have sharp teeth or knurled surfaces that bite into the belt to prevent it from slipping. One way to reduce the chance of belt failure is to mount the entire clamp assembly or the belt anchor on a structural element, such as a bracket, that is designed to deform when the belt force becomes high enough to produce failure. Another way is to limit by suitable stops the amount of movement of the movable jaw toward the fixed jaw so the squeezing force on the belt is kept from becoming high enough to present a risk of belt failure. Both of these approaches, however, are difficult to achieve in reliable, inexpensive and simple ways. In the latter case, strict manufacturing tolerances must be maintained to ensure that the gap between the jaws is stopped at just the right size, lest the squeezing force be too low, thus allowing the belt to slip under too low a tensile load, or too high, thus presenting a risk of belt failure.